Method for making thin film transistor

ABSTRACT

A method for making a thin film transistor, the method comprising the steps of: (a) providing a carbon nanotube array and an insulating substrate; (b) pulling out a carbon nanotube film from the carbon nanotube array by using a tool; (c) placing at least one carbon nanotube film on a surface of the insulating substrate, to form a carbon nanotube layer thereon; (d) forming a source electrode and a drain electrode; wherein the source electrode and the drain electrode being spaced therebetween, and electrically connected to the carbon nanotube layer; and (e) covering the carbon nanotube layer with an insulating layer, and a gate electrode being located on the insulating layer.

RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. §119 fromChina Patent Application No. 200810067163.6, filed on May 14, 2008 inthe China Intellectual Property Office, the contents of which are herebyincorporated by reference. This application is related tocommonly-assigned applications entitled, “THIN FILM TRANSISTOR”,12/384309, filed on Apr. 2, 2009; “METHOD FOR MAKING THIN FILMTRANSISTOR”, 12/384245, filed on Apr. 2, 2009; “THIN FILM TRANSISTOR”,12/384329, filed on Apr. 2, 2009; “THIN FILM TRANSISTOR”, 12/384310,filed on Apr. 2, 2009; “THIN FILM TRANSISTOR PANEL”, 12/384244, filed onApr. 2, 2009; “THIN FILM TRANSISTOR”, 12/384281, filed on Apr. 2, 2009;“THIN FILM TRANSISTOR”, 12/384299, filed on Apr. 2, 2009; “THIN FILMTRANSISTOR”, 12/384292, filed on Apr. 2, 2009; “THIN FILM TRANSISTOR”,12/384293, filed on Apr. 2, 2009; “THIN FILM TRANSISTOR”, 12/384330,filed on Apr. 2, 2009; “METHOD FOR MAKING THIN FILM TRANSISTOR”,12/384241, filed on Apr. 2, 2009; “THIN FILM TRANSISTOR”, 12/384238,filed on Apr. 2, 2009. The disclosures of the above-identifiedapplications are incorporated herein by reference.

BACKGROUND

1. Field of the Invention

The present invention relates to methods for making thin filmtransistors and, particularly, to a method for making a carbon nanotubebased thin film transistor.

2. Discussion of Related Art

A typical thin film transistor (TFT) is made of a substrate, a gateelectrode, an insulation layer, a drain electrode, a source electrode,and a semiconducting layer. The thin film transistor performs aswitching operation. In use, the thin film transistor modulate an amountof carriers accumulated in an interface between the insulation layer andthe semiconducting layer from an accumulation state to a depletionstate, with applied voltage to the gate electrode. Thus, the thin filmtransistor can change an amount of the current passing between the drainelectrode and the source electrode. In practical use, a high carriermobility affect by the material of the semiconducting layer of the thinfilm transistor is desired.

In prior art, the material of the semiconducting layer is amorphoussilicon (a-Si), poly-silicon (p-Si), or organic semiconducting material.The carrier mobility of an a-Si TFT is relatively lower than a p-Si TFT.However, the method for making the p-Si TFT is complicated and has ahigh cost. The organic TFT is flexible but has low carrier mobility.

Carbon nanotubes (CNTs) are a novel carbonaceous material and received agreat deal of interest since the early 1990s. Carbon nanotubes haveinteresting and potentially useful heat conducting, electricalconducting, and mechanical properties. Further, there are two kinds ofcarbon nanotubes: metallic carbon nanotubes and semiconducting carbonnanotubes determined by the arrangement of the carbon atoms therein. Thecarrier mobility of semiconducting carbon nanotubes along a lengthdirection thereof can reach about 1000 to 1500 cm²V⁻¹ s⁻¹. Thus, inprior art, a TFT adopting carbon nanotubes as a semiconducting layer hasbeen produced.

Conventional methods for making a carbon nanotube based TFT includes thesteps of: dispersing an amount of carbon nanotube powder in an organicsolvent to form a mixture; printing the mixture on a substrate;volatilizing the organic solvent to achieve a carbon nanotube layer onthe substrate; forming a source electrode and a drain electrode on thecarbon nanotube layer; forming a silicon nitride layer on the carbonnanotube layer; and forming a gate electrode on the insulating layer.

However, firstly, the carbon nanotubes are prone to aggregate in themixture. Thus, the carbon nanotubes cannot be uniformly dispersed in thecarbon nanotube layer. Secondly, the organic solvent is hard toeliminate from the carbon nanotube layer. Thus, impurities exist in thecarbon nanotube layer. Thirdly, the carbon nanotubes in the carbonnanotube layer are disordered, thus the high carrier mobility of thecarbon nanotube along the length direction thereof cannot be well usedin the TFT. Additionally, the carbon nanotube layer formed by theprinting method is inflexible. Accordingly, the TFT is inflexible.

What is needed, therefore, is a method for making a TFT in which theabove problems are eliminated or at least alleviated.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present method for making the thin film transistorcan be better understood with references to the following drawings. Thecomponents in the drawings are not necessarily drawn to scale, theemphasis instead being placed upon clearly illustrating the principlesof the present method for making the carbon nanotube based thin filmtransistor.

FIG. 1 is a flow chart of a method for making a thin film transistor inaccordance with a first embodiment.

FIG. 2 is a schematic view of the method for making the thin filmtransistor of FIG. 1.

FIG. 3 shows a Scanning Electron Microscope (SEM) image of a carbonnanotube film used in the thin film transistor of FIG. 1.

FIG. 4 is a flow chart of a method for making a thin film transistor inaccordance with a second embodiment.

FIG. 5 is a schematic view of the method for making the thin filmtransistor of FIG. 4.

FIG. 6 is a flow chart of a method for making a thin film transistor inaccordance with a third embodiment.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplifications set out hereinillustrate at least one embodiment of the present method for making thethin film transistor, in at least one form, and such exemplificationsare not to be construed as limiting the scope of the invention in anymanner.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

References will now be made to the drawings to describe, in detail,embodiments of the present method for making the thin film transistor.

Referring to FIG. 1 and FIG. 2, a method for making a thin filmtransistor 10 having a top gate structure is provided in a firstembodiment, and includes the following steps:

-   -   (a) providing a carbon nanotube array and an insulating        substrate 110;    -   (b) pulling out a carbon nanotube film from the carbon nanotube        array by using a tool (e.g., adhesive tape, pliers, tweezers, or        another tool allowing multiple carbon nanotubes to be gripped        and pulled simultaneously);    -   (c) placing at least one carbon nanotube film on a surface of        the insulating substrate 110, to form a carbon nanotube layer        140 thereon;    -   (d) forming a source electrode 151, a drain electrode 152, and a        gate electrode 120;    -   (e) covering the carbon nanotube layer 140 with an insulating        layer 130, wherein the source electrode 151 and the drain        electrode 152 are spaced therebetween, and electrically        connected to the carbon nanotube layer 140, the gate electrode        120 is opposite to and electrically insulated from the carbon        nanotube layer 140 by the insulating layer 130.

In step (a), the material of the insulating substrate 110 can be thesame as a substrate of a print circuit board (PCB), and can be selectedfrom a rigid material (e.g., p-type or n-type silicon, silicon with asilicon dioxide layer formed thereon, crystal, crystal with a oxidelayer formed thereon), or a flexible material (e.g., plastic or resin).In the present embodiment, the material of the insulating substrate ispolyethylene terephthalate (PET). The shape and size of the insulatingsubstrate 110 is arbitrary.

In step (a), a super-aligned carbon nanotube array can be used and isformed by the following substeps: (a1) providing a substantially flatand smooth substrate; (a2) forming a catalyst layer on the substrate;(a3) annealing the substrate with the catalyst layer in air at atemperature approximately ranging from 700° C. to 900° C. for about 30to 90 minutes; (a4) heating the substrate with the catalyst layer to atemperature approximately ranging from 500° C. to 740° C. in a furnacewith a protective gas therein; and (a5) supplying a carbon source gas tothe furnace for about 5 to 30 minutes and growing the super-alignedcarbon nanotube array on the substrate.

In step (a1), the substrate can be a P-type silicon wafer, an N-typesilicon wafer, or a silicon wafer with a film of silicon dioxidethereon. In the present embodiment, a 4-inch P-type silicon wafer isused as the substrate.

In step (a2), the catalyst can be made of iron (Fe), cobalt (Co), nickel(Ni), or any alloy thereof.

In step (a4), the protective gas can be made up of at least one ofnitrogen (N₂), ammonia (NH₃), and a noble gas. In step (a5), the carbonsource gas can be a hydrocarbon gas, such as ethylene (C₂H₄), methane(CH₄), acetylene (C₂H₂), ethane (C₂H₆), or any combination thereof.

The super-aligned carbon nanotube array can be approximately 200 to 400microns in height and include a plurality of carbon nanotubes parallelto each other and approximately perpendicular to the substrate. Thecarbon nanotubes in the carbon nanotube array can be single-walledcarbon nanotubes, double-walled carbon nanotubes, or multi-walled carbonnanotubes. Diameters of the single-walled carbon nanotubes approximatelyrange from 0.5 nanometers to 10 nanometers. Diameters of thedouble-walled carbon nanotubes approximately range from 1 nanometer to50 nanometers. Diameters of the multi-walled carbon nanotubesapproximately range from 1.5 nanometers to 50 nanometers.

The super-aligned carbon nanotube array formed under the aboveconditions is essentially free of impurities such as carbonaceous orresidual catalyst particles. The carbon nanotubes in the super-alignedcarbon nanotube array are closely packed together by van der Waalsattractive force.

In step (b), the carbon nanotube film can be formed by the followingsubsteps: (b1) selecting one or more carbon nanotube having apredetermined width from the super-aligned array of carbon nanotubes;and (b2) pulling the carbon nanotubes to form carbon nanotube segmentsat an even/uniform speed to achieve a uniform carbon nanotube film.

In step (b1), the carbon nanotubes having a predetermined width can beselected by using an adhesive tape such as the tool to contact thesuper-aligned carbon nanotube array. Each carbon nanotube segmentincludes a plurality of carbon nanotubes parallel to each other. In step(b2), the pulling direction is arbitrary (e.g., substantiallyperpendicular to the growing direction of the super-aligned carbonnanotube array).

More specifically, during the pulling process, as the initial carbonnanotube segments are drawn out, other carbon nanotube segments are alsodrawn out end to end due to van der Waals attractive force between endsof adjacent segments. This process of drawing ensures a substantiallycontinuous and uniform carbon nanotube film having a predetermined widthcan be formed. Referring to FIG. 3, the carbon nanotube film includes aplurality of carbon nanotubes joined ends to ends. While there is somevariation, the carbon nanotubes in the carbon nanotube film are allsubstantially parallel to the pulling/drawing direction of the carbonnanotube film, and the carbon nanotube film produced in such manner canbe selectively formed to have a predetermined width. The carbon nanotubefilm formed by the pulling/drawing method has superior uniformity ofthickness and conductivity over a typical disordered carbon nanotubefilm. Further, the pulling/drawing method is simple, fast, and suitablefor industrial applications.

The maximum width of the carbon nanotube film depends on a size of thecarbon nanotube array. The length of the carbon nanotube film can bearbitrarily set as desired (e.g., 1 centimeter to 100 meters). When thesubstrate is a 4-inch P-type silicon wafer, as in the presentembodiment, the width of the carbon nanotube film approximately rangesfrom 0.01 centimeters to 10 centimeters, and the thickness of the carbonnanotube film approximately ranges from 0.5 nanometers to 100 microns.

In step (c), the carbon nanotube layer 140 is used as a semiconductinglayer. It is noted that because the carbon nanotubes in thesuper-aligned carbon nanotube array have a high purity and a highspecific surface area, the carbon nanotube film is adherent in nature.As such, the carbon nanotube film can be directly adhered to the surfaceof the insulating substrate 110. It is noted that, a plurality of carbonnanotube films can be formed in step (b), and stacked and/or placed sideby side on the insulating substrate 110 to form the carbon nanotubelayer 140. Two adjacent carbon nanotube films are combined by van derWaals attractive force therebetween. The aligned direction of the carbonnanotube films is arbitrary. That is, the carbon nanotubes in eachcarbon nanotube film are aligned along a same direction. The carbonnanotubes in different carbon nanotube films can aligned along a samedirection or different directions.

It is noted that, the carbon nanotube layer 140, adhered to the surfaceof the insulating substrate 110, can be treated with an organic solvent.Specifically, the carbon nanotube film can be treated by applyingorganic solvent to the carbon nanotube film to soak the entire surfaceof the carbon nanotube film. The organic solvent is volatilizable andcan, suitably, be selected from the group consisting of ethanol,methanol, acetone, dichloroethane, chloroform, any appropriate mixturethereof. In the present embodiment, the organic solvent is ethanol.After being soaked by the organic solvent, microscopically, carbonnanotube strings will be formed by adjacent carbon nanotubes, that areable to do so, bundling together, due to the surface tension of theorganic solvent. In one aspect, some parts of the carbon nanotubes inthe untreated carbon nanotube film that are not adhered on the substratewill come into contact with the insulating substrate 110 after theorganic solvent treatment due to the surface tension of the organicsolvent. Then the contacting area of the carbon nanotube film with thesubstrate will increase, and thus, the carbon nanotube film can morefirmly adhere to the surface of the insulating substrate 110. In anotheraspect, due to the decrease of the specific surface area via bundling,the mechanical strength and toughness of the carbon nanotube film areincreased and the coefficient of friction of the carbon nanotube filmsis reduced. Macroscopically, the treated film will be approximately thesame uniform carbon nanotube film as the no treated.

In step (d), the material of the source electrode 151, the drainelectrode 152, and the gate electrode 120 has a good conductiveproperty, and can be selected from a group consisting of pure metals,metal alloys, indium tin oxide (ITO), antimony tin oxide (ATO), silverpaste, conductive polymer, and metallic carbon nanotubes. A thickness ofthe source electrode 151, the drain electrode 152, and the gateelectrode 120 is about 0.5 nanometers to 100 microns. A distance betweenthe source electrode 151 and the drain electrode 152 is about 1 to 100microns.

In one embodiment, when the source electrode 151, the drain electrode152, and the gate electrode 120 are made of pure metals, metal alloys,indium tin oxide (ITO), or antimony tin oxide (ATO), a conducting layercan be formed by a depositing, sputtering, evaporating method, andetched to form the source electrode 151 and the drain electrode 152. Inother embodiments, the source electrode 151, the drain electrode 152,and the gate electrode 120 are made of silver paste or conductivepolymer can be formed directly by a print method. In other embodiment,carbon nanotube films with metallic carbon nanotubes therein can beseparately adhered on the substrate or the carbon nanotube layer 140 toform the source electrode 151 and the drain electrode 152, and can beadhered on the insulating layer 130 to form the gate electrode 120.

In the present embodiment, the source electrode 151 and the drainelectrode 152 are separately formed on two ends of the carbon nanotubelayer 140. The carbon nanotubes in the carbon nanotube layer 140 alignedalong a direction from the source electrode 151 to the drain electrode152, to form a carrier channel from the source electrode 151 to thedrain electrode 152.

In the present embodiment, the material of the source electrode 151, thedrain electrode 152, and the gate electrode 120 is pure metal, and step(d) can be performed by a lift-off method or an etching method. Thethickness of the source electrode 151 and the drain electrode 152 isabout 1 micron. The distance between the source electrode 151 and thedrain electrode 152 is about 50 microns.

It is to be understood that, to achieve a semiconducting layer, anadditional step (g) of eliminating the metallic carbon nanotubes in thecarbon nanotube layer 140 can be further performed before step (d). Inone embodiment, the step (g) can be performed by applying a voltagebetween the source electrode 151 and the drain electrode 152, to breakdown the metallic carbon nanotubes in the carbon nanotube layer 140, andthereby achieve a semiconducting layer with semiconducting carbonnanotubes therein. The voltage is in a range from 1 to 1000 volts (V).In another embodiment, the step (g) can be performed by irradiating thecarbon nanotube layer 140 with a hydrogen plasma, microwave, terahertz(THz), infrared (IR), ultraviolet (UV), or visible light (Vis), to breakdown the metallic carbon nanotubes in the carbon nanotube layer 140, andthereby achieve a semiconducting layer with semiconducting carbonnanotubes therein.

In step (e), the material of the insulating layer 130 can be a rigidmaterial such as silicon nitride (Si₃N4) or silicon dioxide (SiO₂), or aflexible material such as PET, benzocyclobutenes (BCB), or acrylicresins. The insulating layer 130 can be depositing, sputtering,evaporating, or printing method according to the material thereof. Athickness of the insulating layer 130 can be in a range from 0.5nanometers to 100 microns.

In the present embodiment, a Si₃N₄ insulating layer 130 is deposited onthe carbon nanotube layer 140, the source electrode 151, and the drainelectrode 152 by a PECVD method. The thickness of the insulating layer130 is about 1 micron.

To be used in a device (e.g., a display), the insulating layer 130 canbe further etched to form exposure holes to expose a part of the sourceelectrode 151, and the drain electrode 152.

Referring to FIG. 4 and FIG. 5, a method for making the thin filmtransistor 20 having a bottom gate structure is provided in a secondembodiment, and is substantially the same as the method form making thethin film transistor 10 in the first embodiment. The main differencebetween the two methods is that the thin film transistor 20 has a bottomgate structure.

The method for making the thin film transistor 20 includes steps of:

-   -   (a′) providing a carbon nanotube array and an insulating        substrate 210;    -   (b′) pulling out a carbon nanotube film from the carbon nanotube        array by using a tool (e.g., adhesive tape, pliers, tweezers, or        another tool allowing multiple carbon nanotubes to be gripped        and pulled simultaneously);    -   (c′) forming a gate electrode 220, a source electrode 251, and a        drain electrode 252;    -   (d′) covering the gate electrode 220 with a insulating layer        230;    -   (e′) laying at least one carbon nanotube film on the insulating        layer 230, to form a carbon nanotube layer 240 thereon; and

wherein the gate electrode 220 is located on a surface of the insulatingsubstrate 210; the source electrode 251 and the drain electrode 252 arespaced therebetween, and electrically connected to the carbon nanotubelayer 230.

The carbon nanotube layer 240 is formed opposite to and insulated fromthe gate electrode 220.

Referring to FIG. 6, a method for making an array of thin filmtransistors is provided in a third embodiment, and is substantially thesame as the method for making the thin film transistor 10 in the firstembodiment. The main difference is that, in the third embodiment, aplurality of thin film transistors is formed in a same substrate,thereby achieving the array of thin film transistors.

The method for making the array of thin film transistors includes stepsof:

-   -   (a″) providing a carbon nanotube array and an insulating        substrate;    -   (b″) pulling out a carbon nanotube film from the carbon nanotube        array by using a tool (e.g., adhesive tape, pliers, tweezers, or        another tool allowing multiple carbon nanotubes to be gripped        and pulled simultaneously);    -   (c″) laying at least one carbon nanotube film on a surface of        the insulating substrate;    -   (d″) patterning the at least one carbon nanotube film, to form a        plurality of carbon nanotube layers thereon;    -   (e″) forming a plurality of pairs of source electrodes and drain        electrodes, and a plurality of gate electrodes separately; and    -   (f″) covering the carbon nanotube layers with the insulating        layers.

wherein the source electrodes and the drain electrodes are spacedtherebetween, and electrically connected to the carbon nanotube layers,the gate electrodes are electrically insulated from the carbon nanotubelayers by the insulating layers.

In step (d″), the at least one carbon nanotube film can be cut by laserbeam, or etched by plasma to form carbon nanotube layers arranged alongcolumns and rows.

In step (e″), a conductive layer can be formed on the whole carbonnanotube layers, and patterned by an etching step to form a plurality ofsource electrodes and drain electrodes separately connected to thecarbon nanotube layers. Further, when the insulating layers covers thecarbon nanotube layers, another conductive layer can be formed on theentire insulating layers, and patterned by an etching step to form aplurality of gate electrodes opposite to the carbon nanotube layers.

In step (f″), an insulating layer can be covered on the whole carbonnanotube layers, source electrodes and drain electrodes, and thenpatterned by an etching step to form a plurality of insulating layerscorresponding to the carbon nanotube layers.

The method for making thin film transistor provided in the presentembodiments comprise the following superior properties. Firstly, thecarbon nanotube film used as the semiconducting layer is simply pulledfrom a carbon nanotube array directly. The carbon nanotubes in thecarbon nanotube film are uniformly dispersed. The purity of the carbonnanotube film is relatively high. Secondly, the carbon nanotube film isadhesive and can be easily adhered in a desired place at a lowtemperature (e.g., room temperature). Thus, the substrate can beselected from a flexible plastic or resin. Thirdly, the carbon nanotubesin the carbon nanotube film are aligned along a same direction andjoined by Van der Waals attractive force. Thus, in the semiconductinglayer of the thin film transistor, the carbon nanotubes can be easilyarranged to align along a direction from source electrode to drainelectrode. Accordingly, the carrier mobility of the thin film transistorcan be improved.

It is to be understood that the above-described embodiments are intendedto illustrate rather than limit the invention. Variations may be made tothe embodiments without departing from the spirit of the invention asclaimed. The above-described embodiments illustrate the invention but donot restrict the scope of the invention.

It is also to be understood that above description and the claims drawnto a method may include some indication in reference to certain steps.However, the indication used is only to be viewed for identificationpurposes and not as a suggestion as to an order for the steps.

1. A method for making a thin film transistor, the method comprising the steps of: (a) providing a carbon nanotube array and an insulating substrate; (b) pulling out a first carbon nanotube film and a second carbon nanotube film from the carbon nanotube array by using a tool, each of the first and second carbon nanotube films comprising a plurality of carbon nanotubes being primarily oriented along the same direction; (c) forming a semiconducting layer by: placing the first carbon nanotube film on a surface of the insulating substrate along a first direction; and stacking the second carbon nanotube film on the first carbon nanotube film along a second direction such that the carbon nanotubes in the first carbon nanotube film are oriented along a different direction than the carbon nanotubes in the second carbon nanotube film; (d) forming a source electrode, a drain electrode, and a gate electrode; and (e) covering the carbon nanotube layer with an insulating layer; wherein the source electrode and the drain electrode are spaced from each other, and electrically connected to the carbon nanotube layer; the gate electrode is located on the insulating layer and electrically insulated from the carbon nanotube layer by the insulating layer.
 2. The method as claimed in claim 1, wherein the plurality of carbon nanotubes are joined end to end by the van der Waals attractive force therebetween.
 3. The method as claimed in claim 2, wherein the carbon nanotubes are semiconducting carbon nanotubes.
 4. The method as claimed in claim 2, wherein the directions of the carbon nanotubes extend substantially from the source electrode to the drain electrode.
 5. The method as claimed in claim 4, wherein the source electrode and the drain electrode are formed directly on the carbon nanotube layer.
 6. The method as claimed in claim 1, further comprising a step (g) of eliminating metallic carbon nanotubes in the carbon nanotube layer.
 7. The method as claimed in claim 6, wherein step (g) further comprises a step of applying a voltage between the source electrode and the drain electrode in order to break down the metallic carbon nanotubes in the carbon nanotube layer.
 8. The method as claimed in claim 6, wherein step (g) further comprises a step of irradiating the carbon nanotube layer with a hydrogen plasma, microwave, terahertz, infrared, ultraviolet, or visible light in order to break down the metallic carbon nanotubes in the carbon nanotube layer.
 9. The method as claimed in claim 1, further comprising an additional step of adhering the carbon nanotube layer to the surface of the insulating substrate by treating the carbon nanotube layer with an organic solvent after step (c).
 10. The method as claimed in claim 1, wherein a material of the insulating substrate is selected from the group consisting of plastic and resin.
 11. The method as claimed in claim 1, wherein in step (e), the insulating layer covers the source electrode and the drain electrode.
 12. The method as claimed in claim 11, wherein step (e) further comprises a step of exposing a part of the source electrode and the drain electrode.
 13. The method as claimed in claim 1, wherein a material of the source electrode, drain electrode, and gate electrode is metallic carbon nanotubes.
 14. A method for making a thin film transistor, the method comprising the steps of: (a′) providing a carbon nanotube array and an insulating substrate; (b′) pulling out a first carbon nanotube film and a second carbon nanotube film from the carbon nanotube array by using a tool, each of the first and second carbon nanotube films comprising a plurality of carbon nanotubes being primarily oriented along the same direction; (c′) forming a gate electrode, a source electrode, and a drain electrode; (d′) covering the gate electrode with an insulating layer; (e′) forming a semiconducting layer by: laying the first carbon nanotube film on the insulating layer along a first direction; and stacking the second carbon nanotube film on the first carbon nanotube film along a second direction such that the carbon nanotubes in the first carbon nanotube film are oriented along a different direction than the carbon nanotubes in the second carbon nanotube film; and wherein the gate electrode is located on the insulating substrate, and the source electrode and the drain electrode are electrically connected to the carbon nanotube layer.
 15. A method for making thin film transistors, the method comprising the steps of: (a″) providing a carbon nanotube array and an insulating substrate; (b″) pulling out a first carbon nanotube film and a second carbon nanotube film from the carbon nanotube array by using a tool, each of the first and second carbon nanotube films comprising a plurality of carbon nanotubes being primarily oriented along the same direction; (c″) adhering a semiconducting layer on the insulating substrate by: placing the first carbon nanotube film on a surface of the insulating substrate along a first direction; and stacking the second carbon nanotube film on the first carbon nanotube film along a second direction such that the carbon nanotubes in the first carbon nanotube film are oriented along a different direction than the carbon nanotubes in the second carbon nanotube film; (g″) treating the carbon nanotube layer adhered on the insulating substrate with an organic solvent; (d″) patterning the first and second carbon nanotube films, to form a plurality of carbon nanotube layers; and (e″) forming a plurality of pairs of source electrodes, drain electrodes, and a plurality of gate electrodes; wherein the source electrodes and the drain electrodes are electrically connected to the carbon nanotube layers, the gate electrodes are electrically insulated from the carbon nanotube layers by the insulating layers.
 16. The method as claimed in claim 15, wherein in step (d″), the first and second carbon nanotube films are cut by laser beam to form the carbon nanotube layers arranged along columns and rows.
 17. The method as claimed in claim 15, wherein in step (d″), the first and second carbon nanotube films are etched by plasma to form the carbon nanotube layers arranged along columns and rows.
 18. The method as claimed in claim 1, wherein the first and second carbon nanotube films are combined by van der Waals attractive force therebetween. 